1. Field of the Invention
The present invention relates to a thin-film transistor and its fabrication method, particularly to a method for fabricating an amorphous-silicon thin-film transistor applicable as an active matrix substrate for a liquid-crystal display.
2. Description of the Prior Art
As for a thin-film transistor (hereafter referred to as TFT) using amorphous silicon for an active layer, it is general to form an N+-type amorphous-silicon layer (hereafter referred to as N+a-Si) doped with high-concentration impurities between amorphous silicon (hereafter referred to as a-Si) and source and drain electrodes as an ohmic contact layer. To improve the contact between the ohmic contact layer and the source and drain electrodes, a transition metal such as Cr, Mo or Ti is normally used.
The official gazette of Japanese Patent Laid-Open No. Sho62-285464 published in 1987 discloses a case of using aluminum (Al) alone as a material of source and drain electrodes on an ohmic contact layer of a TFT. However, it is unavoidable that a natural oxide film is formed on the surface of an N+a-Si serving as an ohmic contact layer during fabrication of a TFT. As proposed in the above described official gazette, even if an electrode made of Al alone is formed on an ohmic contact layer with a natural oxide film on its surface, a preferable contact cannot be obtained. However, when a transition metal such as Cr, Mo or Ti is used, a preferable contact is obtained even if a natural oxide film is present on the surface of N+a-Si. Therefore, an electrode made of Al alone is not actually used though it has an advantage of low resistance.
Thus, when using Al alone or a metal mainly containing Al for source and drain electrodes, a large on-current is not obtained due to a natural oxide film on the surface of an ohmic contact layer. A thin-film transistor showing the above described characteristic has a problem that it cannot be used as an active matrix substrate for a liquid-crystal display.
Therefore, it is an object of the present invention to provide a thin-film transistor using a metal mainly containing Al for source and drain electrodes of an amorphous-silicon TFT so as to be applicable as an active matrix substrate for a liquid-crystal display and its fabrication.
According to the present invention, a gate-insulating layer, a silicon semiconductor layer, and an ohmic contact layer are continuously formed on the entire surface of a transparent insulating substrate so as to cover a gate electrode formed on the substrate to pattern the semiconductor layer and ohmic contact layer in a transistor area like islands. Then, a thin-film-transistor fabrication method is obtained which comprises the steps of performing radio-frequency sputter etching before covering the source and drain electrodes to remove a natural oxide film from the surface of the ohmic contact layer and then, forming a metallic layer mainly containing Al on a gate-insulating layer and the ohmic contact layer.
It is preferable that source and drain electrodes have a double-layer structure configured by an upper layer made of Ti and a lower layer made of Al with an oxide layer formed on the side wall of the structure.
Moreover, it is preferable to use an alloy in which Si is added into Al at a quantity of 0.5 to 3.0 wt % (both included) as a metal mainly containing Al. Furthermore, it is preferable that a silicon semiconductor layer uses an intrinsic a-Si film.
Particularly, according to the present invention, it is preferable to set the time from the step of removing a surface natural oxide film up to start of the step of forming a metallic layer mainly containing Al through sputtering to 1 minute or less.
Moreover, the present invention provides a thin-film transistor fabrication method comprising the steps of forming a gate electrode on a substrate; continuously forming a gate-insulating layer, a semiconductor layer, and an ohmic contact layer on the substrate so as to cover the gate electrode; pattering the semiconductor layer and ohmic contact layer like islands; removing a natural oxide film from the surface of the ohmic contact layer through radio-frequency sputter etching; forming a metallic layer mainly containing aluminum on an ohmic contact layer, an exposed end face of the semiconductor layer, and gate-insulating layer after the step of removing the natural oxide film; and patterning the metallic layer and forming a source electrode and a drain electrode; and further comprising the steps of forming a passivation film so as to cover a thin-film transistor region and a terminal region in which particularly the gate electrode has a double-layer structure configured by a lower-layer metal which is either of a metal mainly containing at least, aluminum and pure aluminum and an upper-layer metal which is a transition metal and the source and drain electrodes have a double-layer structure configured by a lower-layer electrode mainly containing aluminum and an upper-layer electrode made of a transition metal; forming a contact hole for exposing the upper-layer metal to a passivation film on terminal portions of the gate and source electrodes and a passivation film on a region to be connected with a pixel electrode of the drain electrode; and forming a transparent electrode on and around the contact hole to form a gate terminal, a source terminal and a pixel electrode.